The field of this invention relates to smoke-retardant polymer compositions.
The growing concern about the fire safety of consumer products has resulted in an increased recognition that a large percentage of the fatalities resulting from accidental fires can be traced to causes other than burns; namely smoke, heat, noxious gases and deficiency of oxygen. It has been suggested that at least half of the deaths attributed to fire, except for clothing fires, are caused by smoke rather than heat or actual burning. The need to formulate plastic materials to provide a minimum of smoke hazard in an accidental fire is emphasized by the versatility of plastic materials which has led to their use in every conceivable application.
The aspects of smoke from smoldering or burning materials which render smoke hazardous are that smoke is irritating to the eyes and respiratory system; if dense, smoke can impede escape; smoke is toxic, either directly or because of oxygen reduction; smoke can engender panic; smoke constituents can be detrimental to property. Of these hazards, however, only density of smoke has been commonly measured.
During recent years, emphasis has developed upon methods and compositions which will reduce the smoke and flame characteristics of plastic materials. Smoke is known to be made up of fine particles of carbon in a gaseous mixture of the volatilized products of combustion. The common noxious compounds in combustion gases are carbon monoxide, the oxides of nitrogen, hydrogen cyanide, hydrogen sulfide, acetic acid, acrolein, acetaldehyde, formaldehyde, formic acid and ammonia. Other gases are released, depending upon the material being burned.
Polyvinyl chloride (PVC) compositions are inherently self-extinguishing because of the high chlorine content. But, because of this poor burning characteristic, PVC compositions emit large volumes of dark black smoke when burned. The particulate matter of the smoke are carbon particles and partially burned polymer fragments. The noxious gases are principally hydrogen chloride and carbon monoxide but, in the main, hydrogen chloride. It is known that incorporating a smoke-retardant additive into some PVC compositions produces a significant decrease in smoke density and volume.
The flammability of flexible PVC compositions consisting of vinyl chloride polymer, plasticizer, stabilizer, filler and antioxidant is determined largely by the plasticizers used. Aryl phosphates and chlorinated paraffins can be utilized with other plasticizers to give materials flame resistance. However, low temperature properties such as low temperature flexibility and brittle temperature of the resulting PVC composition are affected. If phthalate plasticizers are required to develop necessary properties in the final composition, other fire retardants such as antimony oxide are often used. However, the presence of a fire retardant such as antimony oxide does not necessarily aid in the retardation of smoke. The incorporation of flame retardants into a polymer composition has been found often to cause the emission of increased quantities of smoke by increasing the tendency to smolder.
The usefulness of a smoke suppressant additive depends upon its ability to suppress smoke and also upon whether it will have adverse effects upon the physical properties of the polymer composition. Preferably, smoke retardant additives for vinyl chloride polymers should be sufficiently active in the presence of flame retardants to cause a measurable decrease in smoke output. The smoke retardant additive should be effective within the range of the combustion temperatures of the PVC polymer composition to convert carbonized polymer particles and noxious gases such as carbon monoxide to carbon dioxide. Additionally, the smoke retardant additive should be sufficiently compatible with all constituents of the PVC polymer composition, i.e. resinous polymers, stabilizers, plasticizers -- both primary and secondary, antioxidants, flame retardants, and others, to remain incorporated under conditions of formulation and use. The additive should be sufficiently compatible so that it can be mixed and fused into a coherent, homogeneous material with good stability in that it will not separate easily from the polyvinyl chloride compound when a stress is imposed upon the system such as occurs during extrusion. Chemical stability is an important aspect of compatibility of an additive within the use environment, including conditions of degradation brought about by normal aging. An additive may initially be compatible with a polymer but later exude after a period of use due to degradation of either the additive or the polymer.
Ferrous oxide is known as an oxidative catalyst for carbon monoxide, but ferrous salts of polycarboxylic acids and acid anhydrides have not been known as polymer smoke suppressants. Ferrocene, a coordination compound of ferrous iron and two molecules of cyclopentadiene, has been used as a smoke suppressant additive for vinyl chloride polymers. Ferrocene, however, volatilizes readily at temperatures above 100.degree.C. with a vapor pressure of 2.6 mm Hg at 100.degree.C. Optimum smoke suppressant effect is found at a ferrocene additive level between 0.25 and 0.5%. Unfortunately, various stabilizers are not effective with ferrocene in PVC compositions. Barium-cadminum-zinc phosphite stabilizer systems in PVC compounds can reduce the smoke suppressant effect of the ferrocene additive. Some butyl tin stabilizers when used with ferrocene have been reported to increase smoke in PVC formulations. Ferric smoke suppressants for other polymers are known. Ferric 8-hydroxy-quinolate, as well as the manganese and chromium 8-hydroxyquinolates, has been incorporated into styrene polymers, such as polystyrene, styrene/acrylonitrile copolymers, styrene/butadiene copolymers, styrene/acrylonitrile/butadiene copolymers and other styrene copolymers to reduce smoking.